Polycrystalline Zn insulin has been doped with Cu(II). ESEEM demonstrates two imid coordination to Cu, duplicating the-ray crystal structure. When Cd is added, the EPR and ESEEM of Cu(II) change. Two imid coordination to Cu(II) remains, but the H-bonded water is no loner present, according to the X-ray crystal picture. Serine oxygen is now close to the remote 14N of Cu coordinated imid. the ESEEM spectrum resembles that of galactose oxidase where serine enters in H-bonding to imid.